If this is to support Walmart occasional overnight stops, the stand alone inverter will support you well. MS2000 vs MS2812 is not a large cost jump, and gives you a bit more head room for usage.
If you are going to start boon docking frequently, then also consider adding a good monitoring system at the same time. If going with a Magnum MSxxxx series, then go ahead and get the BMK Meter and ARC-50 remote monitor, perhaps the AGS and SBC components at the same time. Having all components from the same manufacture can simplify install and tech support. If adding Solar in the future too, they have a PT100 controller that also plays well with all of the other components. When adding 4 more 6V's, remember that it's best to have all age wise matched batteries, so you'd be looking at installing 8 6V's.
Back to the Hybrid vs simple PSW Inverter. The Hybrid can also save you some funds if you have a solar set up, and are plugged in for a month at a location that charges you electricity on top of rent space. Some use the Hybrid with their Solar and Battery Bank to split the load between house supplied and metered powers. So it does provide a few more options then just running with a small generator:)!
Good luck to you,
Smitty